Bearing internal clearance is defined as the total distance through which one ring can be moved radially(radial internal clearance) or axially(axial internal clearance) in relation to the other ring under a defined measuring load.

It is necessary to distinguish between the internal clearance of a bearing before it is mounted and the internal clearance of a mounted bearing when in operation(operational clearance).The initial clearance will always be greater than the operational clearance because the rings are expanded or compressed by interferences fits and as a result of the differences in thermal expansion of the bearing rings and mating components. 

The bearing internal clearance referred to as basic has been selected so that when bearings are mounted generally recommended and operate under normal conditions a suitable operational clearance will be obtained. For other conditions, where both rings are mounted with an interference fit or where unusual temperatures prevail, bearings with greater or smaller internal clearance than normal may be required.